Add sealed CI integration: a GitHub Action and a GitLab CI template

scripts/validate.sh is the engine behind both platforms: download the released standalone jar and the validation configuration (SHA-256-verified when pinned), validate each document, print PASS/FAIL with the failing rule ids and fail on any rejection.
The GitHub Action (action.yml) maps its inputs onto the script's environment; the GitLab template fetches the same script from this repository at KOSIT_SCRIPT_REF and drives it with the same variables, so both platforms run identical logic.
With Docker available the action executes each validation sealed (the "sealed" input, default "auto"): inside an eclipse-temurin:21-jre container without network access, a read-only root and no capabilities, with only the jar, the configuration and a scratch directory mounted; downloads always happen outside the container, so the validator itself never touches the network, and the host JVM setup is skipped entirely for "sealed: always".
The image tag pins the Java 21 JRE contract and floats the patch level: the released jars are the checksummed artifacts, the runtime is a security-patch channel; an "image" input (KOSIT_IMAGE on GitLab) accepts a digest-pinned reference for stricter policies.
The GitLab job image is the same runtime, giving both platforms an identical execution environment, and a devcontainer provides the matching Maven and Java setup for development.
The action self-test validates a conformant XRechnung reference instance and asserts a corrupted one is rejected, exercising the sealed path through the empty-input fallback and an explicit image override, plus the host path.

.devcontainer/Dockerfile

@@ -0,0 +1,6 @@
+# Development container for the validator: Maven 3 with Temurin 21, the
+# same Java family the CI action and the GitLab template execute the
+# released jar with (eclipse-temurin:21-jre). The tag pins that contract
+# and floats the patch level, so JDK and Maven security updates arrive
+# without a change to this repository.
+FROM maven:3-eclipse-temurin-21

.devcontainer/devcontainer.json

@@ -0,0 +1,7 @@
+{
+  "name": "KoSIT Validator",
+  "build": {
+    "dockerfile": "Dockerfile"
+  },
+  "postCreateCommand": "mvn --version"
+}